1. Field of the Invention
This invention relates to compositions for coating unvulcanized rubber to prevent adhesion of slabs and/or pellets of such rubber to each other, and aims to provide such compositions which are inexpensive, are easy to apply, produce rapid-drying-non-dusting to lightly dusting films, and which have no harmful effects on the compounded rubber when incorporated into finished products.
2. The Prior Art
Unvulcanized rubber stock is ordinarily used in the forms of slabs which can be handled by an operator. These slabs or pellets will adhere strongly to one another when piled up in heaps, and it is necessary to interpose a non-adhesive film or coating between the slabs or pellets, to prevent this adhesion. Because interposed fabric or plastic film are both expensive and messy to handle, it has been customary to coat the rubber slabs with some material which will form a film or coating on the rubber stock which will prevent this adhesion. The most common materials used are powders, applied as such, or more commonly as aqueous slurries; the powdery deposit left on the rubber spaces the slabs apart, and prevents any adhesive rubber-to-rubber bond from being continuous enough to seriously retard separation of the slabs. Materials such as zinc stearate and mica, widely used in the past, have been generally superseded by kaolin, mainly for cost reasons. The kaolins are generally applied as aqueous slurries in water containing some surfactant, to permit ready spreading of the kaolin particles over the unvulcanized rubber surface.
Two serious problems exist with such materials. Most important, serious dust problems occur in the work areas adjacent the rubber, both in storage and in working up of the rubber in the mills in which they are compounded with other materials. Additionally, aqueous kaolin dispersions tend to settle out, so that the suspensions used to coat the rubber must be stirred regularly to keep them reasonably uniform in composition.
Attempts to improve on aqueous dispersions of Kaolin or other powders have been unattractive for a variety of reasons. Cost is probably the principal cause for rejection of alternatives. Additional reasons are failures to solve the dusting problem, incompatibility with the intended use of the rubber, difficulty in use, or inability to cope with variations in local water supply or rubber stock.